Various efforts are being made at present in various countries with respect to highly efficient power generation systems which employ coal as fuel. For increasing power generation efficiency, it is important to convert the chemical energy of coal into electric energy with high efficiency. However, in recent years how to develop such highly efficient power generation systems has been looked over. The integrated gasification combined cycle (IGCC) technology converts coal into a clean chemical energy by gasification, and then converts the chemical energy directly into electric energy with a fuel cell or utilizes the chemical energy to rotate a gas turbine at a high temperature for generating electric energy with high efficiency. However, since the IGCC technology is oriented toward the complete gasification of the coal, the gasification temperature needs to be increased to a temperature range for melting the ash, resulting in many problems related to the discharge of the molten slag and the durability of the refractory material. Furthermore, because part of the heat energy is consumed for melting the ash, although the generated gases are discharged in such a state that they have a high temperature, the temperature of the generated gases must be lowered for gas purification to a temperature of, for example, about 450° C., causing a very large sensible heat loss. Another problem is that it is necessary to supply oxygen or oxygen-enriched air in order to achieve a high temperature stably. For these reasons, the net energy conversion efficiency is not increased, and no technology is available for generating electric energy with high efficiency using the generated gases. At the present time, it has been found that the net power generation efficiency is not high at all.
In the integrated gasification combined cycle (IGCC), there is an upper limit on the efficiency of the technology for finally converting the chemical energy into electric energy, resulting in a bottleneck in attempts to increase the overall efficiency. Therefore, highly efficient power generation technology that has drawn much attention in recent years simply generates as large an amount of gases as possible while keeping the temperature at the inlet of a gas turbine to an upper limit for increasing the ratio of generated power output from the gas turbine. Typical examples of the highly efficient power generation technology include a topping cycle power generation system and a power generation system using an improved pressurized fluidized-bed furnace.
In the power generation system using an improved pressurized fluidized-bed furnace, coal is first gasified by a pressurized gasification furnace, and generated unburned carbon (so-called char) is combusted by a pressurized char combustor. After combustion gases from the char combustor and generated gases from the gasification furnace are cleaned, they are mixed and combusted by a topping combustor, which produces high-temperature gases to drive a gas turbine. It is important in this power generation system how to increase the amount of gases flowing into the gas turbine. The greatest one of the conditions which imposes limitations on the increase in the flowrate of gases to the gas turbine is the cleaning of the generated gases.
For cleaning the generated gases, it is necessary to cool the generated gases usually to about 450° C. in view of an optimum temperature for a desulfurizing reaction in a reducing atmosphere. On the other hand, the gas temperature at the inlet of the gas turbine should be as high as possible because the efficiency of the gas turbine is enhanced as the gas temperature is higher. At present, it is an ordinal way to increase the gas temperature at the inlet of the gas turbine to 1200° C. or slightly lower due to limitations imposed by heat resistance and corrosion resistance of the materials for the gas turbine. Therefore, the generated gases are required to have a calorific value high enough to increase the gas temperature from 450° C. for the gas cleaning to 1200° C. at the inlet of the gas turbine.
Consequently, for the development of a power generation system using an improved pressurized fluidized-bed furnace, efforts should be made to obtain generated gases in as small an amount as possible and having as high a calorific value as possible. The reasons for those efforts are as follows: If the amount of generated gases to be cleaned at 450° C. is reduced, the loss of sensible heat due to the cooling is reduced, and a required minimum calorific value of the generated gases may be lowered. If the calorific value of the generated gases is higher than the calorific value needed to increase the gas temperature to the required gas temperature at the inlet of the gas turbine, then the ratio of combustion air can be increased to increase the amount of gases flowing into the gas turbine for a further increase in the efficiency of power generation.
In recent years, efforts to develop highly efficient waste combustion power generation technology are being carried out in order to utilize municipal waste, etc. as a fuel. However, one problem of the highly efficient waste combustion power generation technology is that since a high concentration of chlorine may be contained in the waste, the steam temperature for heat recovery cannot be increased beyond 400° C. due to possible corrosion of heat transfer pipes. Therefore, there has been a demand for the development of a technology that can overcome the above difficulty.
One typical conventional gasification furnace which employs coal or the like as a fuel is a twin tower circulation type gasification furnace as shown in FIG. 17 of the accompanying drawings. The two-bed pyrolysis reactor system comprises two furnaces (towers), i.e., a gasification furnace and a char combustion furnace. A fluidizing medium and char are circulated between the gasification furnace and the char combustion furnace, and a quantity of heat required for gasification is supplied to the gasification furnace as the sensible heat of the fluidizing medium which has been heated by the combustion heat of the char in the char combustion furnace. Since gases generated in the gasification furnace do not need to be combusted, the calorific value of the generated gases can be maintained at a high level. However, the two-bed pyrolysis reactor system has not actually been commercialized as a large-scale plant because of problems relating to the handling of high-temperature particles, such as obtaining a sufficient amount of particle circulation between the gasification furnace and the char combustion furnace, controlling of the circulating amount of particles, and stable operation, and problems relating to operation, such as a failure in temperature control of the char combustion furnace independently of other operations.
There has recently been proposed a system in which entire combustion gases discharged from a char combustion furnace are led to a gasification furnace to make up for a shortage of heat for gasification which is not fully supplied by the sensible heat of circulating particles, as shown in FIG. 18 of the accompanying drawings. However, inasmuch as the proposed system delivers the entire combustion gases discharged from the char combustion furnace to the gasification furnace, it goes against the principle of the power generation system using an improved pressurized fluidized-bed furnace that it should be obtained generated gases in as small an amount as possible and having as high a calorific value as possible. That is, if the amount of char combustion gases becomes larger than an amount required for gasification or fluidization in the gasification furnace, then since the generated gases are diluted by the excessive char combustion gases, the calorific value is lowered, and the mixed excessive char combustion gases are also cooled to 450° C. for gas cleaning in a reducing atmosphere, with the result that the quantity of heat necessary to raise the gas temperature to a proper temperature at the gas turbine inlet is increased. Conversely, if the amount of char combustion gases becomes smaller, then the fluidization in the gasification furnace becomes insufficient or the temperature of the gasification furnace is lowered, resulting in a need for supplying air to the gasification furnace. Therefore, in order for this system to be realized, it is necessary to select coal among the limited coal range suitable for the system. If the selected coal differs even slightly from the limited coal range, then the excessive char combustion gases need to be cooled to 450° C., or the calorific value of the generated gases is lowered because of the introduction of air into the gasification furnace, with the result that the efficiency of the overall system will be lowered.
In this system, the temperature of the char combustion furnace is controlled by changing the bed height to change the heat transfer area in the bed. When the system undertakes a low load, as the combustion gases are cooled by the heat transfer pipes exposed over the bed, the temperature of the gasification furnace and the fluidizing gas velocity changes, affecting the gasifying reaction rate to make it difficult to operate the system stably.
In view of the above drawbacks, the inventors of the present invention have devised an integrated gasification furnace comprising a single fluidized-bed furnace which has a gasification chamber, a char combustion chamber, and a low-temperature combustion chamber divided thereby by partitions. The char combustion chamber, the gasification chamber, and the low-temperature combustion chamber are disposed adjacent to each other. The inventors have invented the integrated gasification furnace in order to overcome the drawbacks of the two-bed pyrolysis reactor system described above. The integrated gasification furnace allows a large amount of fluidizing medium to circulate between the char combustion chamber and the gasification chamber. Consequently, heat for gasification can sufficiently be supplied only by the sensible heat of the fluidizing medium. The integrated gasification furnace is possibly able to achieve, most easily, the principle of the power generation system using an improved pressurized fluidized-bed furnace so that generated gases should be obtained in as small an amount as possible and having as high a calorific value as possible.
Nevertheless, the integrated gasification furnace is problematic in that since no complete seal is provided between char combustion gases and generated gases, the combustion gases and the generated gases may be mixed with each other, degrading the properties of the generated gases, if the pressure balance between the gasification chamber and the char combustion chamber is not kept well.
In the field of waste combustion power generation systems, it has been proposed to pyrolyze the wastes and volatilize a chlorine component together with volatile components, and superheat the steam with the combustion heat of remaining char which has a greatly reduced chlorine content, for highly efficient power generation. However, since a small amount of char is produced by the pyrolysis of municipal wastes, it is highly likely not to obtain a char combustion heat required to superheat the steam. The fluidizing medium as a heat medium and the char flow from the gasification furnace into the char combustion furnace, and the same amount of fluidizing medium needs to return from the char combustion furnace to the gasification furnace for achieving a mass balance. According to an available conventional method, the fluidizing medium has to be mechanically delivered by a conveyor or the like, resulting in problems such as the difficulty in handing high-temperature particles and a large sensible heat loss.